Two cents: Stick with Street Fighter. Play on a fightstick if you can. Go for anything else you might fancy afterwards. It's not that Street Fighter is special, or that somehow everything you learn in SF will translate to all other games (it won't in many cases). It's just a good place to start if you're thinking about competitive play at any level. I also recommend playing one of the earlier SF titles (II Turbo and up) before IV.
I don't have any real favorite, but the games I usually play are SF and Melty Blood. can't say I'm any good at them though ._. I have Guilty Gear but I hardly touch it.